Positive Syphilis Serologies after Intravenous Immunoglobulin (IVIG) Treatment: A Diagnostic Confusion That Needs Emphasis

Case Description

The patient under evaluation was a 33-year-old pregnant woman who sought maternal fetal medicine consultation. Her previous neonate developed petechial rash and thrombocytopenia (platelet count: 2.4 × 10¹⁰/L, reference range: 15 to 45 × 10¹⁰/L). Following a referral to genetic counseling, the patient underwent testing for antihuman platelet antigen (HPA)-1a antibody and was positive, while her spouse underwent genetic testing indicating he did not have any variants in the ITGB3 gene encoding HPA-1a. Therefore, alloimmunization was confirmed and her previous neonate was diagnosed with neonatal autoimmune thrombocytopenia (NAIT). Although most cases of NAIT are mild, severe cases can cause intracranial hemorrhage that may result in death or long-term disability (1). Given the high risk of NAIT to the fetus, the patient received prophylactic intravenous immunoglobulin (IVIG) treatment of 1 g/kg/week and prednisone 0.5 mg/kg/day from 20 weeks of gestation, which were increased to 2 g/kg/week and prednisone 0.5 mg/kg/day from 32 weeks of gestation. The patient's early pregnancy screening for syphilis was performed by following a reverse algorithm (described later), which was initially negative for total antibody by enzyme immunoassay (Abbott). However, 6 weeks after starting IVIG treatment, the patient tested positive for syphilis total antibody. Subsequent testing revealed a negative result with the Venereal Disease Research Laboratory (VDRL) test (BD Diagnostics) and a positive result with the Treponema pallidum particle agglutination (TPPA) test (Fujirebio). Additional syphilis tests at 32 weeks of gestation (12 weeks after IVIG initiation) showed positive syphilis total antibody, inconclusive TPPA, and negative VDRL, whereas a test conducted at delivery (37 weeks of gestation or 17 weeks after IVIG initiation) showed positive syphilis total antibody and TPPA and negative VDRL. The IVIG treatment was discontinued at the time of delivery, and a repeat syphilis test performed 6 weeks later returned negative results for syphilis total antibody. TPPA and VDRL tests were not performed as they were not indicated based on the negative syphilis total antibody result.

The patient did not receive syphilis treatment during pregnancy as the positive syphilis tests were interpreted to be false positive. This was due to her negative early pregnancy syphilis screening test, low-risk sexual history, absence of a history of sexually transmitted infections or infectious symptoms throughout her current pregnancy, and the chronological relationship between her IVIG treatment and positive syphilis test results. Such interpretation was confirmed by the negative syphilis total antibody test after IVIG discontinuation.

Case Discussion

IVIG is a blood product that is created by purifying donated blood to obtain super-concentrated human immunoglobulins. The clinical use of IVIG is indicated in various conditions, including humoral immunodeficiency, autoimmune diseases, infectious diseases, or inflammatory states. Depending on the manufacturer, IVIG is manufactured by pooling immunoglobulins derived from 1000 to 100 000 healthy donors (2). Immunoglobulin G (IgG) makes up >90% of the proteins in an IVIG product, which is the principal component required for its therapeutic effects (2). The Food and Drug Administration guidelines recommend that syphilis screening for blood donors be performed using either a nontreponemal or a treponemal test (3). Treponemal tests detect antibodies specifically targeting Treponema pallidum, the causative organism for syphilis, and are highly sensitive and specific for syphilis infection. In comparison, nontreponemal tests measure antibodies produced in response to lipoidal material released by damaged host cells due to Treponema pallidum infection, and therefore lack specificity. Unlike treponemal tests that can remain positive for life, nontreponemal tests may become negative in resolved past infection. Clinically, both tests are used together by following specific algorithms. The reverse algorithm is most commonly used, which begins with a screening treponemal test that, if positive, reflexes to a nontreponemal test. By contrast, the traditional algorithm screens patients using a nontreponemal test that, if positive, is confirmed by a treponemal test. As blood donors may be screened only by a nontreponemal test, those with resolved syphilis infection may be eligible for blood donation after successful treatment, despite their positive treponemal tests. Consequently, the donated blood and the derived IVIG products may contain specific immunoglobulins against treponemal organisms, which can be passively transferred to patients receiving IVIG treatment.

On reviewing the available literature (4–8), it appears that reports of false-positive syphilis serology following IVIG treatment are limited (Table 1). Most reported cases were in pregnant patients, likely due to the higher prevalence of syphilis testing during pregnancy. The reported cases suggest that seroconversion of treponemal tests can occur as soon as 6 days after initiating IVIG (6), but the true time for seroconversion is not known without serially monitored syphilis tests after IVIG initiation. The half-life for IVIG is approximately 3 to 4 weeks (2). For reported cases, the treponemal tests became negative 4 to 7 weeks after discontinuing IVIG, with 4 out of 5 cases becoming negative after 6 to 7 weeks. Therefore, most patients are likely to convert to negative treponemal tests after approximately 2 half-lives of IVIG.

Since commercial IVIG products are manufactured by pooling blood products from numerous healthy donors, there is a high chance that donors with resolved past syphilis infection may be recruited for donation if only a nontreponemal test is used for donor screening. In fact, one study demonstrated that IVIG from different manufacturers were found to be consistently positive for treponemal tests but negative for nontreponemal tests (8). Therefore, it is crucial to identify false-positive syphilis tests in patients who have received IVIG to avoid unnecessary treatment, emotional distress, and social scrutiny. This is especially important during pregnancy because congenital syphilis is a devastating but treatable disease and clinicians may be more likely to start syphilis treatment for pregnant patients who develop positive treponemal tests. In addition, the treponemal-specific IgG can be passively transferred through the placenta and result in a false-positive treponemal test in the fetus (6). Benzathine penicillin G is the recommended treatment for early syphilis during pregnancy and although it is a generally safe antibiotic to be used for both pregnant woman and infants, there is still a risk for an allergic reaction that should be avoided whenever possible.

Owing to the passive transfer of immunoglobulins, IVIG-related false-positive syphilis tests should theoretically be positive only for treponemal tests. Among the 6 reviewed cases, 5 patients developed positive treponemal tests after IVIG, while their nontreponemal tests remained negative. Therefore, the development of an isolated positive treponemal test by following a reverse algorithm after IVIG initiation is an important clue to recognize IVIG-related false-positive syphilis tests. Although the false-positive treponemal tests can be avoided by following a traditional algorithm, the reverse algorithm should still be used as the preferred syphilis testing during pregnancy due to its ability to identify more late latent infections, which are still transmissible to fetus and can cause serious congenital damage. It should be noted that false-positive nontreponemal tests, albeit at a low titer of 1:2, could also occur after IVIG and complicate result interpretation (4). It is well-established that false-positive nontreponemal tests can be associated with various medical conditions unrelated to syphilis so a positive nontreponemal test for a patient receiving IVIG therapy should be interpreted in the context of the patient’s entire medical history.

Documenting a negative syphilis treponemal test before IVIG treatment is also helpful in demonstrating the chronological relationship between IVIG treatment and the subsequent positive treponemal test result. Moreover, obtaining relevant social and sexual history can help clinicians to better assess the risk of newly onset syphilis vs passive transfer of treponemal-specific immunoglobulins. To confirm a false-positive treponemal test result, a follow-up test should be obtained at least 6 weeks after IVIG discontinuation. This step can provide further confirmation and help rule out the possibility of an actual syphilis infection.

In addition to false-positive syphilis serology, passive transfer of IgG following IVIG administration may affect other serology tests that are commonly performed during pregnancy. In our patient, we evaluated the effect of IVIG treatment on total immunoglobulins (IgG, IgA, and IgM) and serology tests for various other pathogens using the blood samples obtained during IVIG treatment and after its discontinuation (Table 2). The disparity in index values and interpretation between IVIG treatment and post-discontinuation underscores the broad impact of IVIG therapy on serology results. This raises concerns about the potential for generating false-positive results, which could subsequently lead to unnecessary follow-up testing or treatment.

In summary, false-positive syphilis serology tests following IVIG treatment are a common yet often overlooked occurrence that can pose diagnostic challenges in various clinical settings, particularly during pregnancy. It is important for both pathologists and clinicians to be aware of this phenomenon to ensure accurate result interpretation. Conducting pre- and post-IVIG treponemal testing and identifying an isolated positive treponemal test can typically resolve diagnostic uncertainties. Testing each lot of IVIG products for the presence of treponemal-specific antibodies before infusion can also help to confirm the false-positive treponemal tests after IVIG treatment. Finally, IVIG manufacturers may also want to consider screening all blood donors using treponemal tests to prevent misinterpretation of syphilis tests in patients receiving IVIG treatment.

Ethical Approval

This study is approved by the Institutional Review Board of University Hospitals Cleveland Medical Center (STUDY20221663).

Nonstandard Abbreviations

HPA, antihuman platelet antigen; NAIT, neonatal autoimmune thrombocytopenia; IVIG, intravenous immunoglobulin; VDRL, Venereal Disease Research Laboratory; TPPA, Treponema pallidum particle agglutination.

Author Contributions

The corresponding author takes full responsibility that all authors on this publication have met the following required criteria of eligibility for authorship: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; (c) final approval of the published article; and (d) agreement to be accountable for all aspects of the article thus ensuring that questions related to the accuracy or integrity of any part of the article are appropriately investigated and resolved. Nobody who qualifies for authorship has been omitted from the list.

Mingfei Yan: data curation—equal, formal analysis—equal, investigation—equal, writing—original draft—lead, writing—review and editing—lead; Susan Zhang: data curation—equal, formal analysis—equal, investigation—equal, writing—review and editing—equal; and Jaime Noguez: conceptualization—lead, data curation—equal, formal analysis—equal, methodology—lead, supervision—lead, writing—review and editing—lead.

Authors’ Disclosures or Potential Conflicts of Interest

Upon manuscript submission, all authors completed the author disclosure form.

Research Funding

None declared.

Disclosures

J. Noguez is SYCL Liaison to The Journal of Applied Laboratory Medicine, Association for Diagnostics & Laboratory Medicine.

References